WANG Wei（王 維），LI Xuan（李 璇），PAN Qunli（潘群力）

School of Reliability and Systems Engineering，Beihang University，Beijing 100191，China

Introduction

In recent years，with the science and technology development and the population increasing，escalators have been an indispensable transport equipment closely related to our lives.The escalator accidents let us have to summarize the risk factors that could be avoided［1］.The box elevator belongs to enclosed elevator.Also most of the risk analyses are discussed about enclosed elevator，and escalator is rarely mentioned.But on July 5th，2011in Beijing subway line 4 escalators gained our focus.The subway is a travel hub，and the traffic is very big.The subway escalators are heavy haul of escalator related to the people's life safety，but they still have varieties of risks.Risk analysis can assess risk factors and provide risk aversion［2］.

After so many years of research and development，escalator has been a mature product.There are some large escalator companies in the world，such as Otis，Hitachi and Mitsubishithat still spend a lot of manpower and material resources for escalators and moving sidewalks development.Since escalator has been an indispensable part of public life，escalator research mainly focuses on the energy-saving，environmental protection，and safety.Security becomes one of the key parts when compared with all of them［3］.This paper reports on risk management of the escalator overturned accident，and we aim to find the hazard about escalator and avoid the accident happened again.

1 Identification and Analysis of Hazard

The purposes of the escalator risk analysis are to establish security systems and use the principle and method of safety system engineering to identify and qualitatively and quantitatively analyze the risk factors existing in the escalator system，then put up the optimal scheme of escalator system security.The process is to use the method to test，inspect，and analyze through the escalator risk assessment，forecast the security situation in the life cycle including the danger，accident，probability and severity of failure，and take measures to reduce risk.

1.1 Factors related to the hazard identification

There are two main factors that lead to danger：people's unsafe behaviors and states.The dangerous accidents will happen when aforementioned two main factors coexist.For these two reasons and defects in management，analyzing the escalator system and considering dangerous occurrence are required.

1.1.1 Physical states of escalator systems

The physical states of escalator systems include escalators system structure，physical state of the system，its subsystems of staircase and so on.According to the escalator system structures and the escalator industry habits，standards and regulations as well as functional description convenience，escalator hazards can be divided into ladder system， main transmission system，tensioner，handrail system，comb plate，truss，electric system，safety protection system，etc.Physical state of escalator system and its corresponding subsystem include geometry， material properties，component characteristics，motion and so on.Escalator system consists of seven subsystems，and their function and the main components have been shown in Table 1.

Table 1 Function and composition of escalator subsystems

1.1.2 Behaviour and management

Personnel related to the escalators are divided into the following kinds： installation personnel， using personnel，homework personnel near the escalator，and maintenance management personnel.

Furthermore，escalator system safety related factors can be summarized as escalator equipment，operations management，service management，and related personnel，as shown in Fig.1.

Fig.1 Escalator safety factors

There are many types of escalator accidents，for example， falling from the escalator， the escalator overturned，crash and shear with the objects，and falling down.For aforementioned accidents， the escalator overturned has received a lot attention these years.Therefore，the risk management of the escalator overturned accident based on the factors of risk is studied carefully in this paper.

1.2 Identificating hazard

Fishbone diagram analysis method，to analyze and summarize the various causes of accident，is described by concise text and lines which can be used to indicate the reasons and results.Fishbone diagramnot only can identify and eliminate the“root cause”of a problem systematically，but also can prevent problems from happening again.

Generally，qualitative analysis escalator overturned accident by fishbone diagram can help understandthe main reasons of the escalator overturned accident which is an important step in the hazard identification.Fishbone diagram analysis is shown in Fig.2.

Fig.2 Escalator overturned accident analysis using fishbone diagram

2 Risk Analysis

On the basis of risk identification，qualitative and quantitative analyzing risk factors（hazards）and the risk of loss that causes the risk events （accidents or disasters）provide the basis for risk assessment［4］.There are many kinds of risk analysis methods and fault tree analysis（FTA）methods in the paper.

FTA can not only analyze the proximate cause of the accident，but also further suggest the potential causes of the accidents.It is an effective method for risk analysis，prediction，and control accident.

FTA consists of several steps shown in Fig.3.

2.1 FTA of escalator overturned accident

Reversal refers to the escalator in the cases of loss of electric drive and control，and the escalator driving direction change.Once the phenomenon happens，it leads to the personnel's psychological fear，or even causes casualties andescalator equipment damage［5］.

According to the structure of the escalator and the results of the related accident analysis，the reasons of the accident can be summerized as follows.

Escalator does not meet the normal traction conditions which leads to the lack of traction.Power grid fault phase，low voltage，and outage cause motor reversal or driving force；low voltage components or devices short-circuit，adhesion，and open circuit fault lead to escalator overturned；defect of parts；drive actuator；cascade and the slippage of the tape drivefault lead to the traction and braking force insufficiency or missing.

Fig.3 Process of FTA

Brake control failures and insufficient braking force cause brake failure.Safe circuit has the function of the brake control circuit which therefore cannot provide the protection，control，and stop.Brake motion components by reason of manufacture，installation，wear and tear，failure，which will lead to insufficient braking or missing［6］.

Escalator bears too much of people，and serious overloading causes the lack of motor torque which leads to decline.

By analyzing the causes of escalator overturned accident，combining with the logical relationship between them，F(xiàn)TA is shown in Fig.4.

Fig.4 FTA of escalator overturned accident

According to the logical relationship between various factors in the FTA，its structure function can be expressed as：

2.2 Results analysis

2.2.1 Accident tree's cut set

Accident tree's cut set，a collection of basic events that causes the top event，is the model of the system fault.The minimum cut sets at the bottom of the events is the minimum quantity and essential.All of minimum cut sets represent all the faults of the system，and it can be used to describe the system all the basic failure mode.

There are many methods to determine the minimum cut set.The commonly used methods are ranks method and Boolean algebra method.By using Boolean algebraic method，there are 90 minimum cut sets：｛X3，X10｝，｛X3，X15，X18｝，｛X3，X15，X19｝，｛X3，X15，X20｝，…，｛X1，X11｝，｛X1，X12｝，｛X1，X16｝，｛X1，X17｝，｛X1，X2｝.There are 60order minimum cut sets in the analysis above.At the same time，the minimum cut sets appear three times that also has a larger threat to system security.The minimum cut set for long-term unchecked is located which contains the maximum numbers of basic events，so each minimum cut set rarely occurs at the same time relatively which is the preferred way to prevent and control the accident.According to the fault tree's minimum cut set，we can know that there are many failure modes of escalator overturned accident，and all of them are easy to happen.

After simplifying the accident tree，its structure is shown in Fig.5which is equivalent of fault tree diagram.

According to the equivalent diagram，we can see that escalator overturned has 90possible paths.For example，the critical path of subway line 4shows one of the minimum cut sets is｛X1，X8｝，and the probability of escalator overturned accident is to multiply the results that are then added by 90kinds of each basic event in every minimum cut set.So the probability of accident is greater than P｛X1，X8｝，and P｛X1，X8｝＝PX1，PX8.Furthermore，the escalator overturned accident in the subway line 4.Otis escalator design defects have been the determining factors in the accident report，so P｛X1，X8｝depends on the possibility of component damage directly.As a result，taking attention to component damage is very important in daily check.

Fig.5 Equivalent of fault tree diagram

2.2.2 Minimal path set

Opposite to the minimum cut sets in the fault tree，a group of basic events does not occur，and then the top event will happen，where this set of basic events is called path set.The more paths we get，the more ways to prevent the top event happening will be.In practical engineering applications，the path set which includes the least number of basic events is chosen，and basic events are controlled to avoid accident.Or we can choose the most economic path set，which can reduce the cost significantly.

The structure function of the success-tree is given by：

There are many success-tree's minimum cut sets：｛X′3，X′4，X′5，X′6，X′7，X′8，X′13，X′14，X′9，X′1｝，｛X′10，X′15，X′11，X′12，X′16，X′17，X′2｝，｛X′10，X′18，X′19，X′20，X′11，X′12，X′16，X′17，X′2｝，and they are also the fault-tree's minimal path sets.According to the results，no checking for a long time should be the more prior than braking force.Opening the under voltage and gate opening stuck are required.Namely，we should give priority to ensure the basic events in the path set｛X′10，X′15，X′11，X′12，X′16，X′17，X′2｝do not occur.

The fault tree's structural formula expressed in the path set is：

Fault tree is drawn with the minimal path set method and shown in Fig.6.

Fig.6 Equivalent diagram of the fault tree

From the success-tree of the escalator overturned accident，we can see that there are three minimum caliber assemblages.As long as the basic events in each of the minimal paths set don't occur，we are able to control the accident not to happen.The more minimum caliber assemblage in the system，the more security.There will be several kinds of control schemes after getting the whole minimum caliber assemblage.Therefore，we can draw up three kinds of treatment schemes that avoid the accident.From the three minimum caliber assemblages，the event P2contains the least amount of basic events，so P2should be the optimal way to control escalator overturned accident.

3 Escalator Engineering Risk Assessment and Risk Control

3.1 Risk assessment

Risk assessment，after the hazard identification and risk analysis，should be given to assess the risk.There are several kinds of evaluation methods，and the LEC evaluation method to assess the escalator overturned accident is used in this paper.

The basic idea of LEC evaluation method is that the value of the personnel casualty risk assessment depends on three factors related to the object of risk assessment.Three factors aforementioned are as follows.

1）L：the possibility of accidents.Expressed in probability normally， the probability of absolutely impossible event is 0，while the probability of certain events is 1.When using LEC method，absolutely impossible and certain events are not needed to be noticed.The accident possibility is scored from 10to 0.1，and there are a number of intermediate values between the upper and the lower values.

2）E：how often the body is exposed to dangerous environments.The more higher frequency and the longer time that workers exposed to dangerous environment，the more greater the risk will be，and the value is between 10 and 0.5.

3）C：the consequences of the accidents，which means that the loss when the accident happens.The range of the loss is very wide；the most serious accident can lead to many deaths；some accident causes minor injuries，thus the value of Cis provided by a large range，from 100to 1.

The value of D can be calculated as：D ＝LEC.

Evaluation values of L，E，and C are given according to the principles shown in Tables 2-4［7］.

Table 2 Possibility of the accident（L）

Table 3 Frequency exposed to dangerous environment（E）

Table 4 Consequences of the accident（C）

According to the value of D，we can determine the monitoring and corrective actions quickly.According to experience，a score below 20is considered as low risk.It's more safety than riding a bike to work everyday；if risk score is between 70-160，it is a significant risk which needs rectification timely；if the risk score between 160-320，immediate measures must be taken to improve security；if the score is higher than 320，the environment is very dangerous， then the production should be stopped immediately until the environmental problems are solved.The values of LEC about the escalator overturned accident are as follows.

1）L：escalators are widely in the subway，especially in recent years，escalators have developed rapidly in our country，and productions are continuing to grow.However，the escalator overturned accidents don't happen frequently，so L ＝3.

2）E：the escalators are used in the working time，so E＝6.

3）C：the accident can cause serious consequences，1-2 people died and many people injury，so C ＝15.

The assessment value of the project accident risk without taking measures can be calculated as D＝LEC＝3×6×15＝270.

The level of the project risk is level 2（the value of Dis between 160and 320），which is very dangerous.Therefore，it needs to take measures to reduce the risk of accidents and improve the safety conditions immediately.

To sum up，the escalator overturned accident belongs to high-risk situations；more attention to the daily maintenance in normal operation conditions should be paid.

3.2 Risk control

After risk identification， risk analysis and risk assessment，we know much information about the projects or system risk as well as its gap with risk management objectives，and find the problems needed to be solved in order to ensure the security of life.According to risk analysis，control measures should be done as follows.

1）Escalator companies not only take the routine maintenance according to the standard escalator，but also need to check the components that connect to the mainframe and the stair regularly，where are pure mechanical parts，which must be repaired before fracture，crack and other clear signs，and be solved before the accident happened.

2）There are design defects in the protection device of the escalator overturned accident，so modify the protection device design to improve the reliability is required.

3）Popularize protection knowledge of the escalator overturned accident is necessary.

4 Conclusions

The suitable methods for escalator risk analysis based on risk theory is presented，and the proposed method is combined with FTA，LEC method and so on.The goal of comprehensive evaluation is achieved.The basic conclusions are given as follows.

1）Different types of escalator accidents have different causes of the accident and the corresponding hazards.There are hazards inescalator normal work.In this paper，the research lays a foundation for hazard identification accurately.

2）This paper identifies the detailed hazard according to the hazard identification and risk rank，the technical support for the establishment of safety control measures is provided.

3）The ultimate purpose of hazard management is to achieve effective control of hazards，the principles of scientific，representative，independence and operability.The technology and management method to identify hazards clearly is proposed which can evaluate risk and control the accident effectively.

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